It sounds like he doesn't have the correct audio settings, he needs to go into his sounds setting an check is output/input on his pc. If he can hear yall his output is correct he just needs to change where his voice is picked up from(input), this can be done in the sound setting, might have to hit advance settings depending on os version w10 make it a bit weird. Once the above is done, he needs to go into his mixer settings, different then sound settings, this will be a small pop up window an it will have all his devices that can pull audio in/out with bars that measure the volume, one tab is playback the other is record, he needs to go into record an make sure his mic is registered as default an enabled, windows love to do some stupid -blam!- someone times. It helpfully to be in a fireteam with him so he can switch between input device till you can hear him, if his buy himself he'll have to guess an wont know till he get in a team. After he has his settings have him check on it every few weeks, windows loves to do some stupid -blam!- sometimes an can reset all these settings on an update sometime